#define
by itself will replace the symbol with nothing.
On the other hand, #define 1
, as you call it, will replace the symbol with 1
everywhere it is found in the file. So, for example, the following code:
#include <iostream>
#define EXAMPLE "hello"
int main()
{
std::cout << EXAMPLE;
return 0;
}
prints
hello
This is because EXAMPLE
here is replaced with "hello"
, making the print statement equivalent to:
std::cout << "hello";
If we change the #define
statement to this instead:
#define EXAMPLE
This will give a compile error:
main.cpp: In function ‘int main()’:
main.cpp:15:25: error: expected primary-expression before ‘;’ token
std::cout << EXAMPLE;
As to why you would ever use this second form of #define
, it's because there is another processor directive that you can use called #ifdef
:
#include <iostream>
#define EXAMPLE
int main()
{
#ifdef EXAMPLE
std::cout << "EXAMPLE defined.";
#endif
return 0;
}
This will print:
EXAMPLE defined.
Because #ifdef
(and its relative #ifndef
) only require that the symbol be defined, we don't really need to give it a value. It just needs to be there to work.
A common place you see this kind of stuff is with header guards (which is probably what you're seeing). You can also see it with platform identification, or even to determine whether the compiler is using C++ or not.